C. M. Boese

Extreme hydrothermal conditions at an active plate-bounding fault

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealands transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7–35 Hz) have been recorded on shallow borehole seismometers installed within 20 m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60–200 m with a 10–40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

ABSTRACT During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.

Microseismicity and P–wave tomography of the central Alpine Fault, New Zealand

ABSTRACT We utilise seismic data from the central section of the Alpine Fault to locate earthquakes and image crustal structure in three dimensions. Tomography results from c. 6500 sources reveal the fault as either a southeast-dipping low-velocity zone or a marked velocity contrast in different parts of the study region. Where our model is best resolved, we interpret the Alpine Fault to be listric in nature, dipping steeply in the upper crust (50–60°) and flattening to 25–30° in the lower crust. The base of the seismogenic zone shallows from c. 15 km beneath the footwall and Alpine Fault to c. 6 km beneath the Southern Alps Main Divide, and then deepens to c. 15 km by c. 10 km further southeast. The shallow brittle–ductile transition overlies a broad low-velocity zone, which together likely result from the presence of fluids and elevated temperatures brought about by enhanced exhumation rate in this section of the Alpine Fault.

Background and delayed‐triggered swarms in the central Southern Alps, South Island, New Zealand

Low-magnitude earthquake swarms (ML ≤ 2.8), consisting of up to 47 events of similar waveforms, have been observed repeatedly in the central Southern Alps, a rapidly uplifting orogen bounded by the transpressive Alpine Fault in the South Island of New Zealand. We compare nine background swarms recorded between November 2008 and April 2010 with five delayed-triggered swarms that occurred after the MW 7.8 Dusky Sound and the MW 7.1 Darfield (Canterbury) earthquakes. The two types of swarms are similar in terms of the magnitudes, depths, focal mechanisms, and interevent times of the constituent microearthquakes, and appear to both involve the rupture of steeply dipping faults in highly fractured crust in a 10 km × 12 km area in the center of the SAMBA network. The delayed-triggered swarms occurred at similar epicentral distances (c. 4.5× the rupture length of the mainshocks) to the Dusky Sound and Darfield earthquakes, commenced shortly after the passage of the surface waves, continued for ∼5 and ∼2 days, respectively, and were followed in each case by a ≥2 day long quiescent period, which may suggest clock-advanced of faults in their failure-cycle. Triggering thresholds of ≥0.01 MPa proposed elsewhere are similar to the dynamic stress changes computed for the Southern Alps (≥0.09 MPa). However, as 98% of the locatable triggered events occurred several hours after the surface waves had passed, the dynamic stress changes associated with the surface waves themselves are unlikely to have triggered the earthquakes directly. Instead, we suggest that the locations and delays of the triggered swarms are more consistent with triggering by pore pressure diffusion.

Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the faults principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

Implications of upper-mantle seismicity for deformation in the continental collision zone beneath the Alpine Fault, South Island, New Zealand

ABSTRACT We review the state of knowledge regarding lower-crustal and upper-mantle deformation in the continental collision zone beneath the Alpine Fault in the central South Island. Existing lithospheric deformation models based on a variety of geophysical observations and different interpretations of tectonic reconstructions range from intra-continental subduction to lithospheric mantle thickening. We derive independent information in the 40–80 km depth range from a new catalogue of 78 upper mantle earthquakes, after almost a decade’s observations (2006–2016). The events occur in both the Australian and Pacific plates, are clustered in distinct locations that coincide with positive magnetic anomalies and follow an inferred structural trend differing from the current plate boundary orientation. For two event clusters 13 new well-constrained focal mechanism solutions are used for stress field inversion. This yields a common, sub-horizontal maximum compressive stress orientation (120° ± 27° and 120° ± 22°) but suggests different intermediate and minimum principal stress orientations. Based on this we cannot unequivocally distinguish published mantle deformation models but we can evaluate certain model features. Strong upper-mantle anisotropy together with a VP-VS-ratio of 1.73 ± 0.08 for upper-mantle earthquakes suggests that a proposed eclogite layer is unlikely to be present at upper mantle depths. We conclude that seismicity at depths ≥40 km reflects inherited heterogeneous strength distributions in the upper mantle and delineates areas in which deformation occurs along a (structural) convergence axis in the continental collision zone.